Center flex single side polishing head

ABSTRACT

A polishing head assembly for single side polishing of silicon wafers includes a polishing head and a cap. The polishing head includes a top surface and a bottom surface and defines a longitudinal axis extending therethrough. The cap is positioned coaxially with the polishing head and includes an upper surface and a lower surface. The upper surface is spaced from the bottom surface of the polishing head to form a chamber that allows the cap to deflect toward the polishing head.

CROSS REFERENCE

This application claims priority to U.S. Provisional Application No. 61/829,685 filed May 31, 2013, the disclosure of which is hereby incorporated by reference in its entirety.

FIELD

This disclosure relates generally to polishing of semiconductor or solar wafers and more particularly to single side polishing apparatus and methods for controlling flatness of the wafer.

BACKGROUND

Semiconductor wafers are commonly used in the production of integrated circuit (IC) chips on which circuitry are printed. The circuitry is first printed in miniaturized form onto surfaces of the wafers. The wafers are then broken into circuit chips. This miniaturized circuitry requires that front and back surfaces of each wafer be extremely flat and parallel to ensure that the circuitry can be properly printed over the entire surface of the wafer. To accomplish this, grinding and polishing processes are commonly used to improve flatness and parallelism of the front and back surfaces of the wafer after the wafer is cut from an ingot. A particularly good finish is required when polishing the wafer in preparation for printing the miniaturized circuits on the wafer by an electron beam-lithographic or photolithographic process (hereinafter “lithography”). The wafer surface on which the miniaturized circuits are to be printed must be flat. Similarly, flatness and finish are also important for solar applications.

The construction and operation of conventional polishing machines contribute to the unacceptable flatness parameters. Polishing machines typically include a circular or annular polishing pad mounted on a turntable or platen for driven rotation about a vertical axis passing through the center of the pad and a mechanism for holding the wafer and forcing it into the polishing pad. The wafer is typically mounted to the polishing head using for example, liquid surface tension or a vacuum/suction. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad for greater polishing interaction between the polishing pad and the surface of the wafer. This type of polishing operation is typically referred to as chemical-mechanical polishing (CMP).

During operation, the pad is rotated and the wafer is brought into contact with and forced against the pad by the polishing head. As the pad wears, e.g., after a few hundred wafers, wafer flatness parameters degrade because the pad is no longer flat, but instead has a worn annular band forming a depression along the polishing surface of the pad. Such pad wear impacts wafer flatness, and may cause “dishing” or “doming” or a combination thereof resulting in a “w-shape”.

When the flatness of the wafers becomes unacceptable, the worn polishing pad has to be replaced with a new one. Frequent pad replacement adds significant costs to the operation of the polishing apparatus not only because of the number of pads that need to be purchased, stored, and disposed of, but also because of the substantial amount of down time required to change the polishing pad.

Accordingly, there is a need for a polishing apparatus that has the ability to optimize flatness parameters by modulating the wafer thickness shape in the polishing process for doming, dishing, and +/−w-shape.

This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

SUMMARY

A first aspect is a polishing head assembly for single side polishing of silicon wafers. The polishing head assembly includes a polishing head and a cap. The polishing head includes a top surface and a bottom surface and defines a longitudinal axis extending therethrough. The cap is positioned coaxially with the polishing head and includes an upper surface and a lower surface. The upper surface is spaced from the bottom surface of the polishing head to form a chamber that allows the cap to deflect toward the polishing head. The wafer carrier is disposed adjacent to the cap.

Another aspect is a single side polishing apparatus for single side polishing of silicon wafers. The single side polishing apparatus includes a polishing head for rotating the wafer in relation to a polishing pad, a cap, and a movable stopper. The polishing head has a polishing side facing a location of the wafer. The cap is mounted in spaced relation from the polishing side of the polishing head and is capable of deflecting under a polishing pressure. The movable stopper is located between the polishing head and the cap to limit movement of the cap with respect to the polishing head along a direction that is perpendicular to the polishing side of the polishing head.

Another aspect is a method for the polishing of silicon wafers. The method includes providing a polishing apparatus that includes a wafer carrier for retaining a wafer placed therein and a polishing head assembly. The polishing head assembly has a polishing head and a cap attached to a polishing side of the polishing head for rotating the wafer in relation to a polishing pad. The polishing head is moved with respect to the wafer carrier to cause the cap to react against the wafer resulting in a polishing pressure. The cap is deflected with respect to the polishing head. A surface of the wafer is polished by causing rotational movement between the wafer and the polishing pad to form a polished surface on the wafer. The polishing pressure is adjusted to regulate the deflection of the cap for improving flatness of the polished surface.

Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic elevation of a single side polisher;

FIG. 2 is a cross section of a single side polishing head with a domed plate in accordance with one embodiment, the shape of the domed plated being exaggerated for illustration purposes;

FIG. 3 is a cross section of the single side polishing head of FIG. 3 under polishing pressure;

FIG. 4 is a cross section of the single side polishing head of FIG. 3 under polishing pressure 0.9 P;

FIG. 5 is a cross section of the single side polishing head of FIG. 3 under polishing pressure 1.1 P;

FIG. 6 is a pressure profile graph plotting the correlation of the contact pressure and wafer radius;

FIG. 7 is a cross section of a single side polishing head with a flat plate in accordance with another embodiment;

FIG. 8 is a cross section of the single side polishing head of FIG. 7 with a pressurized section under no polishing pressure;

FIG. 9 is a cross section of the single side polishing head of FIG. 7 with a pressurized section under polishing pressure;

FIG. 10 is a cross section of the single side polishing head of FIG. 7 with a pressurized section under polishing pressure 0.9 P;

FIG. 11 is a cross section of the single side polishing head of FIG. 7 with a pressurized section under polishing pressure 1.1 P;

FIG. 12 is a cross section of a single side polishing head with a flat plate and two adjustable stoppers in accordance with another embodiment;

FIG. 13 is a cross section of the single side polishing head of FIG. 12 with a pressurized section under no polishing pressure;

FIG. 14 is a cross section of the single side polishing head of FIG. 12 with a pressurized section under polishing pressure;

FIG. 15 is a cross section of the single side polishing head of FIG. 12 with a pressurized section under polishing pressure 0.9 P;

FIG. 16 is a cross section of the single side polishing head of FIG. 12 with a center stopper retracted and a pressurized section under polishing pressure 1.3 P;

FIG. 17 is a cross section of the single side polishing head of FIG. 12 with stopper ring retracted and a pressurized section under polishing pressure 1.3 P;

FIG. 18 is a cross section of the single side polishing head of FIG. 12 with both stoppers retracted and a pressurized section under polishing pressure 1.1 P;

FIG. 19 is a Doming Correction graph plotting the correlation of the contact pressure and the radius;

FIG. 20 is a Negative W-factor Correction graph plotting the correlation of the contact pressure and the radius; and

FIG. 21 is a cross section of a single side polishing head with a flat plate and an inflatable bellow attached to a center stopper in accordance with another embodiment.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Generally, and in one embodiment of the present disclosure, a wafer that has previously been rough polished so that it has rough front and back surfaces is first subjected to an intermediate polishing operation in which the front surface of the wafer, but not the back surface, is polished to improve flatness parameters or to smooth the front surface and remove handling scratches. To carry out this operation, the wafer is placed against the polishing head. In this embodiment, the wafer is retained in position against the polishing head by surface tension. The wafer also is placed on a turntable of a machine with the front surface of the wafer contacting the polishing surface of a polishing pad.

A polishing head mounted on the machine is capable of vertical movement along an axis extending through the wafer. While the turntable rotates, the polishing head is moved against the wafer to urge the wafer toward the turntable, thereby pressing the front surface of the wafer into polishing engagement with the polishing surface of the polishing pad.

A conventional polishing slurry containing abrasive particles and a chemical etchant is applied to the polishing pad. The polishing pad works the slurry against the surface of the wafer to remove material from the front surface of the wafer, resulting in a surface of improved smoothness. As an example, the intermediate polishing operation preferably removes less than about 1 micron of material from the front side of the wafer.

The wafer is then subjected to a finish polishing operation in which the front surface of the wafer is finish polished to remove fine or “micro” scratches caused by large size colloidal silica, such as Syton® from DuPont Air Products Nanomaterials, LLC, in the intermediate step and to produce a highly reflective, damage-free front surface of the wafer. The intermediate polishing operation generally removes more of the wafer than the finishing polishing operation. The wafer may be finish polished in the same single-side polishing machine used to intermediate polish the wafer as described above. However, a separate single-side polishing machine may also be used for the finish polishing operation. A finish polishing slurry typically has an ammonia base and a reduced concentration of colloidal silica is injected between the polishing pad and the wafer. The polishing pad works the finish polishing slurry against the front surface of the wafer to remove any remaining scratches and haze so that the front surface of the wafer is generally highly-reflective and damage free.

Referring to FIG. 1, a portion of a single side polishing apparatus is shown schematically and indicated generally at 100. The single side polisher 100 may be used to polish a front surface of semiconductor wafers W. It is contemplated that other types of single side polishing apparatus may be used.

The polishing apparatus 100 includes a wafer holding mechanism, e.g., a backing film 110, a retaining ring 120, a polishing head assembly 130, and a turntable 140 having a polishing pad 150. The backing film 110 is located between a polishing head assembly 130 and the retaining ring 120, which receives a wafer W. The retaining ring 120 has at least one circular opening to receive the wafer W to be polished therein. The wafer W of this embodiment is retained against the polishing head assembly 130 by surface tension.

The polishing apparatus 100 applies a force to the polishing head assembly 130 to move the polishing head assembly vertically to raise and lower the polishing head assembly 130 with respect to the wafer W and the turntable 140. An upward force raises the polishing head assembly 130, and a downward force lowers the polishing head assembly. As discussed above, the downward vertical movement of the polishing head assembly 130 against the wafer W provides the polishing pressure to the wafer to urge the wafer into the polishing pad 150 of the turntable 140. As the polishing apparatus 100 increases the downward force, the polishing head assembly 130 moves vertically lower to increase the polishing pressure.

A portion of the polishing head assembly 130 and polishing pad 150 and turntable 140 are rotated at selected rotation speeds by a suitable drive mechanism (not shown) as is known in the art. The rotational speeds of the polishing pad and the turntable may be the same or different. In some embodiments, the apparatus 100 includes a controller (not shown) that allows the operator to select rotation speeds for both the polishing head assembly 130 and the turntable 140, and the downward force applied to the polishing head assembly.

With reference to FIG. 2, the polishing head assembly 130 includes a polishing head 160 and a cap 170. The cap is suitably made of plastic, aluminum, steel, ceramic, such as alumina or silicon carbide, or any suitable material with sufficient stiffness, including coated silicon.

The cap 170 includes a plate or floor 172 surrounded by an annular wall 174 extending upward therefrom. In a natural or un-deflected state, the floor 172 has a concave shape (relative to the chamber), such that the center of the floor is lower than the perimeter. The floor 172 is capable of temporarily deflecting without permanently deforming. The floor 172 is about 0.118 to about 0.275 inches (3-7 mm) thick, or about 0.625 inches (16 mm) thick for plastic, and has a diameter of about 5.905 to about 6.496 inches (150-165 mm).

The annular wall 174 is rigidly attached to and extends downward from an edge 162 of the polishing head 160. Together the polishing head 160 and the cap 170 form a downwardly domed structure. The cap 170 may be attached to the polishing head 160 with bolts or other suitable fasteners. In other embodiments, an adhesive, such as an epoxy, is used to attach the cap 170 to the edge 162 of the polishing head 160.

Downward movement of the polishing head assembly 130 causes the cap 170 to contact the wafer W, and deflects the floor 172 upward toward the polishing head 160. The direction of deflection is perpendicular to the top surface of the wafer W.

As the polishing pressure of the cap 170 against the wafer W increases, the magnitude of deflection also increases. Regulation of the polishing pressure allows the deflection of the floor 172 to be increased or decreased. As the deflection of the floor 172 is changed, the shape of the floor is also changed.

Changing the shape of the floor 172 causes a resulting change in the force distribution of the polishing pressure across the wafer W and thereby causes the wafer to bend in response. The change in force distribution also causes a change in the rate of removal of material from the wafer W. Generally, the rate of removal is increased at portions of the wafer W that transfer relatively greater force to the polishing pad 150.

As a result, the downward force of the polishing head assembly 130 may be controlled to increase or decrease the deflection of the floor 172 of the cap 170 and thereby adjust the amount of doming or dishing of the wafer. As the polishing pressure is increased, the floor 172 transitions from a natural, un-deflected or downwardly curved shape to a flat shape that is substantially parallel with a bottom surface of the polishing head 160, and finally to an upwardly curved or convex shape.

As shown in FIG. 3, under a given or predetermined polishing pressure P the floor 172 is deflected to be substantially flat, resulting in a removal profile that is also substantially flat. As shown in FIG. 4, lowering the polishing pressure 0.9 P causes both the floor 172 and the removal profile to become downwardly curved. As shown in FIG. 5, increasing the polishing pressure 1.1 P causes both the shape of the floor 172 and the removal profile to become dished. Suitably, the change in polishing pressure may range from about 0.7 P to about 1.3 P. Thus, a change in polishing pressure P provides an operator with a control variable and the ability to adjust the polished shape of the wafer W. In some embodiments, the predetermined polishing pressure may range from 1.0 psi to 4.0 psi. In other embodiments, the predetermined polishing pressure may be less than 6.0 psi.

In some embodiments, the domed shaped plate may be attached to an existing polishing head so as to change the polishing properties of the polisher without extensively reworking the machine or buying a new machine.

With reference to FIG. 6, a plot of a finite element simulation showing the correlation between the contact pressure and the radius are shown. This plot illustrates the ability of this embodiment to modulate the contact pressure profile by increasing or decreasing the standard polishing pressure P resulting in an adjustment of the removal profile.

With reference to FIG. 7, another embodiment of the polishing head assembly 230 has a polishing head 260 and a cap 270. The cap 270 includes a floor 272 surrounded by an annular wall 274. The floor 272 is substantially flat in an initial or un-deflected state. The annular wall 274 is rigidly attached to and extends downward from an edge 262 of the polishing head 260 to form a chamber 232 between the floor 272 and the polishing head 260. The chamber 232 may be connected with a pressurizing source (not shown) to provide a pressurizing media or fluid to the chamber 232.

As shown in FIG. 8, the chamber 232 may be pressurized causing the floor 272 to deflect into a downward dome shape, similar to floor 172. The pressure within the chamber 232 does not need to be changed frequently. Therefore, it may be adjusted manually when the polishing head assembly 230 is mounted on the polishing apparatus 100. In some embodiments, the cap 270 may be used to retrofit an existing polishing head without the need to drill holes through the existing head or spindle for passage of air or fluid through the spindle and rotary unions.

Similar to the floor 172, the floor 272 of the cap 270 is adapted to temporarily deflect in a direction that is perpendicular to the polishing surface as the polishing pressure is increased. The cap is not permanently deflected or deformed by the pressure. The floor 272 transitions from a pressurized deflected or downwardly curved shape to a flat shape that is substantially parallel with a bottom surface of the polishing head 260, and finally to a upwardly curved or convex shape as the polishing pressure is increased.

As shown in FIG. 9, under a given or predetermined polishing pressure P the floor 272 is deflected to be substantially flat, resulting in a removal profile that is also substantially flat. As shown in FIG. 10, lowering the polishing pressure 0.9 P causes both the floor 272 and the removal profile to become downwardly curved. As shown in FIG. 11, increasing the polishing pressure 1.1 P causes both the shape of the floor 272 and the removal profile to become dished. Thus, a change in polishing pressure P provides an operator the ability to adjust the polished shape of the wafer W.

As described above, the polishing system is capable of adjusting pressure distribution to control the shape of a polished wafer, e.g., for minimizing doming and dishing of the wafer after polishing.

With reference to FIG. 12, another embodiment of the polishing head assembly 330 is adapted to adjust pressure distribution to a wafer during the polishing process for controlling or minimizing doming, dishing, and w-shape cross section of the wafer.

The polishing head assembly 330 has a polishing head 360, a cap 370, a stopper ring 380, and a center stopper 382. The cap 370 includes a floor 372 surrounded by an annular wall 374. The floor 372 is substantially flat in an initial or un-deflected state. The annular wall 374 is rigidly attached to and extends downward from an edge 362 of the polishing head 360 to form a chamber 332 between the floor 372 and the polishing head 360. The chamber 332 may be connected with a pressurizing source (not shown) to provide a pressurizing fluid to the chamber 332.

As shown in FIG. 13, the chamber 332 may be pressurized causing the floor 372 to deflect into a downward dome shape, similar to floors 172 and 272. The pressure within the chamber 332 does not need to be changed frequently. Therefore, it may be adjusted manually when the polishing head assembly 330 is mounted on the polishing apparatus 100. In some embodiments, the cap 370 may be used to retrofit an existing polishing head without the need to drill holes through the existing head for passage of air or fluid through the spindle and rotary unions.

Similar to the floors 172 and 272, the floor 372 of the cap 370 is adapted to temporarily deflect in a direction that is perpendicular to the polishing surface as the polishing pressure P is increased. The cap is not permanently deflected or deformed by the pressure. The floor 372 transitions from a pressurized deflected or downwardly curved shape to a flat shape that is substantially parallel with a bottom surface of the polishing head 360, and finally to a upwardly curved or convex shape.

As shown in FIG. 14, under a given or predetermined polishing pressure P, the floor 372 is deflected to be substantially flat, resulting in a removal profile that is also substantially flat. As shown in FIG. 15, reducing the polishing pressure 0.9 P causes both the floor 372 and the removal profile to become downwardly curved.

Stopper ring 380 and center stopper 382 limit the upward deflection of the floor 372. The stopper ring 380 is spaced inward from the outer circumference and is annular in shape. The center stopper 382 is co-axially aligned with the polishing head 360, the cap 370, and the stopper ring 380. With additional reference to FIG. 12, the stopper ring 380 and the center stopper 382 extends from the bottom surface of the polishing head 360 to an inner or top surface of the floor 372. The height of the stopper ring 380 and the center stopper 382 are substantially equivalent to the between the polishing head 360 and the floor 372.

With reference to FIGS. 15-18, the height of each the stopper ring 380 and the center stopper 382 may be adjusted to vary the deflection shape of the floor 372.

As shown in FIG. 16, reducing the height of the center stopper 382 and increasing the polishing pressure 1.3 P causes both the shape of the floor 372 and the removal profile to become w-shaped. As shown in FIG. 17, reducing the height of the stopper ring 380 and increasing the polishing pressure 1.3 P causes both the shape of the floor 372 and the removal profile to become m-shaped. As shown in FIG. 18, reducing the height of both the stopper ring 380 and the center stopper 382, and increasing the polishing pressure 1.1 P causes both the shape of the floor 372 and the removal profile to become domed. Thus, a change in polishing pressure provides the operator with the ability to adjust the polished shape of the wafer W.

Finite element simulation results illustrating this embodiment's ability to modulate contact pressure profiles for adjusting doming and w-shape wafer profiles are shown in FIGS. 19 and 20. FIG. 19 is a doming correction graph that plots the correlation of the contact pressure and the radius in accordance with the embodiment above is shown. FIG. 20 is a negative w-factor correction graph that plots the correlation of contact pressure and the radius in accordance with the embodiment above is shown.

With reference to FIG. 21, another embodiment of the polishing head assembly 430 is adapted to adjust pressure distribution to a wafer during the polishing process for controlling or minimizing doming, dishing, and w-shape cross section of the wafer.

The polishing head assembly 430 has a polishing head 460, a cap 470, and a center stopper 480. The cap 470 includes a floor 472 that is rigidly attached to edges 462 of the polishing head 460 with screws 464 and extends across the edges to form a chamber 432 between the floor and the polishing head. In other embodiments, an adhesive is used to attach the floor 472 to the edges 462 of the polishing head 460, instead of screws 464. The floor 472 is substantially flat in an initial or un-deflected state. The chamber 432 is connected with a first pressurizing source FS through a chamber passageway 466 and connector 486 to provide a pressurizing fluid to the chamber 432 that may cause the floor 472 to deflect into a downward dome shape, similar to floors 172, 272, and 372.

Similar to the floors 172, 272, and 372, the floor 472 of the cap 470 is adapted to temporarily deflect in a direction that is perpendicular to the polishing surface as the polishing pressure is increased. The cap 470 is not permanently deflected or deformed by the pressure. The floor 472 has the ability to transition from a pressurized deflected or downwardly curved shape to a flat shape that is substantially parallel with a bottom surface of the polishing head 460, and finally to an upwardly curved or convex shape based on the amount of pressurizing fluid supplied to the chamber 432.

Under a given or predetermined polishing pressure P, the floor 472 is deflected to be substantially flat, resulting in a removal profile that is also substantially flat. Reducing the polishing pressure 0.9 P causes both the floor 472 and the removal profile to become downwardly curved.

The center stopper 480 includes a stop 482 connected with inflatable bellows 484. The bellow 484 is connected with and extends into the chamber 432 from the polishing head 460. The height of the stop 482 is adjusted by increasing or decreasing the pressure within the bellows 484, which is connected with a second pressurizing source SS through a center passageway 468 and a connector 488. Adjustment of the pressure within the bellows 484 may either limit the upward deflection of the floor 472 or to deflect the floor 472 outward. The first and second pressurizing sources FS and SS may be connected through a controller 490 to respective connectors 486 and 488. In some embodiments, first and second pressurizing sources FS and SS are the same pressurizing source and are connected with the respective connectors 486 and 488 through the controller 490, which may include a divider and control valves (not shown). In some embodiments, the polishing head assembly 430 does not include a center stopper 480. In these embodiments, the first pressurizing source FS may be supplied to the chamber 432 through the center of the polishing head 460.

The center stopper 480 is co-axially aligned with both the polishing head 460 and the cap 470. During operation, the center stopper 482 may extend from the bottom surface of the polishing head 460 to an inner or top surface of the floor 472, such that the height of the center stopper 480 is substantially equivalent to the chamber between the polishing head 460 and the floor 472.

In a method of one embodiment, a polishing process of a wafer is controlled by adjustment of the polishing pressure used in the polishing apparatus 100 to change the shape of a polishing head assembly to regulate the polished shape of a wafer W. In some embodiments, the polishing apparatus is a single side polisher. In another embodiment, the polishing pressure is regulated by a controller before or during the polishing process.

The method includes providing a polishing apparatus with a turntable for rotating a polishing pad in relation to the wafer and a polishing head assembly with a cap attached to a polishing side of a polishing head. The cap is moved with respect to the wafer to cause a polishing pressure from the wafer to react against the cap. The cap is deflected with respect to the polishing head. A liquid may be applied to the cap to wet it, such that the wafer is held in place, against the cap, by surface tension when the wafer is placed against it. The liquid on the lower surface of the cap can be squeezed to almost zero film thickness to retain the wafer by surface tension.

The polishing pressure reacting against the cap is adjusted to regulate the deflection of the cap for improving flatness of the polished surface. A surface of the wafer is polished by causing movement between the wafer and the polishing pad to form a polished surface on the wafer. An internal pressure within a chamber between the polishing head and the cap may be adjusted to regulate deflection of the cap with respect to the wafer for improving flatness of the polished surface.

Changing the shape of the cap from dish to dome or vice versa changes the material removal profile of the wafer and thus, changes the shape of the polished wafer. A domed head will cause more material removal in the center and thus make the wafer thickness profile dished relative to its thickness profile before polishing. While a dished head will remove more material on the edges of the wafer making the wafer domed relative to its thickness profile before polishing. In some embodiments, the shape of the polishing head may be changed by adding a low stiffness cap to the bottom of the existing head and then regulating the polishing pressure to deflect the cap to change its shape.

The embodiments described herein provide the ability to modulate the polishing head for doming, dishing, and +/−w-shape to enable an efficient and economical polishing method of processing semiconductor wafers. The method improves wafer yield and process capability, while reducing product tolerances and the time needed for maintenance associated with the replacement of the polishing pads and templates mounted on the single side polishing head.

When introducing elements of the present invention or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, “down”, “up”, etc.) is for convenience of description and does not require any particular orientation of the item described.

As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense. 

What is claimed is:
 1. A polishing head assembly for single side polishing of silicon wafers, the polishing head assembly comprising: a polishing head having a top surface and a bottom surface and defining a longitudinal axis extending therethrough; and a cap positioned coaxially with the polishing head, the cap having an upper surface and a lower surface, the upper surface being spaced from the bottom surface of the polishing head to form a chamber therebetween to allow the cap to deflect toward the polishing head.
 2. The polishing head assembly of claim 1, further comprising a source of pressurized fluid connected with the chamber to increase a pressure within the chamber and to deflect the cap.
 3. The polishing head assembly of claim 1, further comprising a controller connected with the source of pressurized fluid to control the pressure within the chamber.
 4. The polishing head assembly of claim 1, further comprising a liquid on the lower surface of the cap to retain a wafer by surface tension.
 5. The polishing head assembly of claim 4, further comprising a wafer connected with the cap, the wafer being retained against the lower surface of the cap by surface tension.
 6. The polishing head assembly of claim 1, wherein the polishing head assembly is configured to cause movement of the polishing head along the longitudinal axis to increase and decrease a polishing pressure.
 7. The polishing head assembly of claim 1, further comprising a first stopper located within the chamber, between the polishing head and cap to limit deflection of the cap at a first location, the first stopper having a first height.
 8. The polishing head assembly of claim 7, further comprising a second stopper located within the chamber, between the polishing head and cap to limit deflection of the cap at a first location, the first stopper having a second height.
 9. The polishing head assembly of claim 8, wherein the second stopper is a ring coaxially located about the first stopper.
 10. The polishing head assembly of claim 8, wherein the first height is one of less than and greater than the second height.
 11. The polishing head assembly of claim 8, wherein the chamber has a chamber height, the chamber height is greater than at least one of the first height and the second height.
 12. A single side polishing apparatus for single side polishing of silicon wafers, the single side polishing apparatus comprising: a polishing pad; a polishing head for forcing the wafer toward the polishing pad, the polishing head having a polishing side facing a location of the wafer; a cap mounted in spaced relation from the polishing side of the polishing head, the cap being capable of deflecting toward the polishing head under a polishing pressure and away from the polishing head by an increase of pressure in the chamber. a movable stopper located between the polishing head and the cap to limit deflection of the cap along a direction that is perpendicular to the polishing side of the polishing head.
 13. The single side polishing apparatus of claim 12, further comprising a source of pressurized fluid connected with the chamber to increase pressure within the chamber and to deform the cap.
 14. The single side polishing apparatus of claim 12, further comprising a second stopper located between the polishing head and the cap at a position radially outward from the movable stopper to limit deflection of the cap.
 15. The single side polishing apparatus of claim 12, wherein the cap is attached to the polishing head with an adhesive.
 16. The single side polishing apparatus of claim 14, wherein the floor is flat in an undeflected state.
 17. The single side polishing apparatus of claim 12, further comprising a movable stopper located between the polishing head and the cap to limit deflection of the cap along a direction that is perpendicular to the polishing side of the polishing head.
 18. The single side polishing apparatus of claim 17, further comprising a source of pressurized fluid connected with the stopper to adjust pressure within the stopper and to regulate a height of the stopper.
 19. A method for polishing of silicon wafers; the method comprising: providing a polishing apparatus including a polishing pad, a wafer carrier for retaining a wafer placed therein, and a polishing head assembly having a polishing head and a cap attached to a polishing side of the polishing head; forcing the polishing head toward the polishing pad to cause the wafer to contact the polishing pad at a polishing pressure; polishing a surface of the wafer by causing rotational movement of the polishing pad with respect to the wafer; deflecting the cap with respect to the polishing head; and adjusting the polishing pressure to regulate the deflection of the cap for improving flatness of the polished surface.
 20. The method of claim 19, further comprising adjusting an internal pressure within a chamber formed between the polishing head and the cap to deflect the cap for improving flatness of the polished surface.
 21. The method of claim 19, further comprising placing the wafer adjacent to the cap and retaining the wafer with surface tension to the cap.
 22. The method of claim 19, wherein the polishing head includes a stopper to limit the deflection of the cap.
 23. The method of claim 22, further comprising adjusting the height of the stopper to regulate the deflection of the cap.
 24. The method of claim 23, wherein adjusting the height of the stopper includes the step of adjusting pressure within the stopper. 